Intensity modulated radiation therapy (IMRT) verification using the frame averaging feature of Varian Mark II electronic portal imaging device (EPID)

The authors have previously developed a dose-based IMRT verification system, but its usefulness is limited by the image acquisition and storage time of the Varian PoralVision EPID. In this study, the authors investigate the applicability of the EPID´s frame-averaging feature to acquire ~1 sec images and averages over them before storage. However, the frame-averaged image cannot be converted to a dose map due to the nonlinearity of the EPID response. To circumvent this, the authors convert the dose rate from intensity-modulated (IM) beams to EPID readings and then sum over the simulated ~1 sec images to produce the intended frame-averaged image (IFAI). Frame-averaged images (EFAI) are measured with an EPID for the same IM fields. The authors also simulated frame-averaged image (SFAI) by acquiring a series of individual images under the normal mode and summing them offline. The IFAI, EFAI and SFAI are then compared using a linear regression method, whose standard deviation of errors, σ, is a measure of the goodness of match. Artificial malfunctions were also introduced into IM beam delivery to test the sensitivity of this approach. The results indicate that SFAI agrees with IFAI with a mean σ=3% and a standard deviation (SD)<0.6%. EFAI, however, shows less agreement with IFAI with a mean σ=4.1% and a SD of 1.5%. Most, but not all, simulated malfunctions were detectable using EFAIs. It is concluded that the frame-averaging feature of Varian Mark II EPID shows promise for treatment verifcation, but that more investigation is required to validate this approach for clinical application